The CDC says that though the outbreak seems to be over, "additional cases from the outbreak period could still be identified."

CDC investigators have interviewed people who ate at Taco Bell and didn't get sick and those who became infected by E. coli.

The CDC's analysis of those interviews suggests that three items -- shredded lettuce, cheese, or ground beef -- were eaten more often by people who fell sick.

Taco Bell's cheese is pasteurized and its beef is cooked. Those processes should kill E. coli bacteria, leaving lettuce as the prime suspect.

Lettuce Link?

The FDA is following the lettuce lead, looking for the farm or farms that grew the iceberg lettuce and any facilities where the lettuce was processed.

If lettuce was the E. coli source, contamination probably happened before reaching the restaurants, says the CDC.

Green onions had earlier been considered a possible E. coli source. But onions of any type are not linked to this outbreak, according to the CDC.

Taco Bell Plans Improvements

"I want to reassure our customers that it is absolutely safe to eat at Taco Bell," says Taco Bell President Greg Creed in a Taco Bell news release.

Creed announced this week that the company plans to lead an industry coalition of government regulators, competitors, suppliers, and other experts to develop improved guidelines and procedures to safeguard the produce supply chain and public health.

"Going forward, we commit to our customers to work with our internationally recognized experts in food safety as we lead an industry effort to improve produce safety standards at the farm level to prevent this from happening again at our restaurant, or wherever customers may buy their food," Creed says.

When the outbreak began, Taco Bell switched its supplier for all produce, including lettuce, for its restaurants in Delaware, New Jersey, New York, and Pennsylvania.

Taco Bell says it also discarded food and resanitized all affected restaurants in the region.